Conventional biodiesel production employs homogeneous alkaline catalysts to transform seed oils or animal fats into fatty acid alkyl esters and glycerin. The normal volume ratio of alkyl esters to glycerin is 10:1. Separating the glycerin from the ester layers by capitalizing on their different specific gravities—1.26 kg/L for glycerin and 0.86-0.90 kg/L for esters—is common but cost inefficient.
Large quantities of water are required to remove glycerin and spent catalyst from the ester layer, which tends to reduce the market value of the glycerin byproduct. Static or centrifugal separators are difficult to manage and tedious to operate, lending considerable risk to the quality of the final alkyl ester product, which must meet ASTM specifications (D6751-07b) before any use in on-road vehicles as biodiesel.
Newer continuous processes for biodiesel production using heterogeneous catalysts enable the transeterification reaction to proceed continuously. Such continuous processing requires the application of cost effective, time efficient, and complete separation of glycerin from the alkyl ester stream. Because no water is used in these newer solid catalytic processes, the quality of the glycerin is higher (about 98%) and its market value considerably greater than glycerin from homogeneous catalytic processes. The lower volume glycerin streams, which typically range from less than 400 barrels per day to as much as 1,000 barrels per day, require a continuous, rapid separation for their economy.
Recent tests conducted by the inventors have shown that glycerin can be readily and rapidly coalesced by an electrostatic field and the separation rate is increased by the development of large glycerin droplets. Although electrostatic coalescence is a proven, effective method for crude oil dehydration, electrostatic coalescers are not well-suited for biodiesel production. These crude oil coalescers are typically large, horizontally oriented vessels. A need exists, therefore, for smaller, vertically oriented, electrostatic coalescers to promote the separation of glycerin from alkyl fatty acid esters in the continuous production of biodiesel.